1. Field of the Invention
The invention relates in general to a semiconductor manufacturing method, and more particularly to a method of forming a complementary metal oxide semiconductor (CMOS) sensor.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a portion of a semiconductor device showing a conventional complementary metal oxide semiconductor (CMOS) sensor.
In FIG. 1, the P-type well 101 is located on the substrate 100. The first oxide layer 102 and the second oxide layer 103 are located on the P-type well 101. The first oxide layer 102 and the second oxide layer 103 define the active region 104. The gate oxide layer 105 is located in the active region 104. The gate conductive layer 106 is located on the gate oxide layer 105. The first spacer 107 and the second spacer 108 are located on the side walls of the gate conductive layer 106. The first lightly doped drain region 109 and the second lightly doped drain region 110 are located below the first spacer 107 and the second spacer 108 in the P-type substrate 101. The first source/drain region 112 is located between the first lightly doped drain region 109 and the first field oxide layer 102 in the P-type substrate 101. The second source/drain region 113 is used as a depletion region.
Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting light into an electrical signal that represents the intensity of the energy. The applications of CCDs include monitors, transcription machines and cameras. Although CCDs have many strengths, CCDs also suffer from high costs and the limitations imposed by the CCDs' volume. To overcome the weaknesses of CCDs and reduce costs and dimensions, a CMOS photodiode device is developed. Because a CMOS photodiode device can be produced using conventional techniques, costs and the volume of the sensor can be reduced. The applications of CMOS photodiodes include PC cameras, digital cameras etc.
The photodiode is based on the theory that a P-N junction can convert light into an electrical signal. Before energy in the form of photons strikes the photodiode, there is an electric field in the P-N junction. The electrons in the N region do not diffuse forward to the P region and the holes in the P region do not diffuse forward to the N region. When enough light strikes the photodiode, the light creates a number of electron-hole pairs. The electrons and the holes diffuse forward to the P-N junction. When the electrons and the holes reach the P-N junction, as a result of the effect of the inner electric field across the junction, the electrons flow to the N region and the holes flow to the P region. Thus a current is induced between the P-N junction electrodes. Ideally, a photodiode in the dark is open-circuit. In other words there is no current induced by light while the photodiode is in the dark. After forming the photo diode, subsequent steps, such as forming a metal interconnect and forming an inter-layer dielectric layer, are performed to complete a photo device structure.
Inter-metal dielectric (IMD) layers or inter-layer dielectric (ILD) layers are generally used to separate and electrically isolate wiring lines and other conductors in semiconductor circuit devices. As devices are being scaled down to smaller geometries, such devices may include multiple layers of wiring lines and other conductors and require isolation between adjacent conducting structures and isolation between layers. Total thickness of the IMD layers or of the ILD layers becomes thicker than before. Thick dielectric material above the sensor region of the CMOS sensor causes interference when light passes through the dielectric material into the sensor region and decreases the sensitivity of the CMOS sensor.